1. Field of the Invention
The present invention relates to a polymer liquid crystal emulsion, having improved dispersibility of the liquid crystal and stability of the dispersed liquid crystal by imparting part of a hydrophobic property to a polymer. It also relates to a nematic curvilinear alignment phase film using the polymer liquid crystal emulsion produced by the present invention, and to a method for producing the same.
2. Description of the Related Art
As a medium for transmitting various kinds of information in the Age of Information, a liquid crystal display (hereinafter referred to as "LCD") is one of the most widely used apparatuses together with a cathode ray tube (hereinafter referred to as "CRT"). However, LCDs adopting a twisted nematic (hereinafter referred to as "TN") mode or a super twisted nematic (hereinafter referred to as "STN") mode which have been most widely used, necessitate the use of dispersing agents to control the size of the dispersed liquid crystals and to improve the stability of the liquid crystal. Therefore, the development of a new polymer matrix to be used in LCDs has become urgently sought.
The development of the polymer dispersion liquid crystal (referred to as "PDLC") composite films for liquid crystal display elements arose from using liquid crystals in new polymer material fields having electro-optical properties. The method for preparing PDLC composite films necessitates a phase separation procedure, so that low molecular weight nematic crystal liquids can be dispersed evenly in a polymer matrix. There are three phase separating methods, polymerization induced phase separation, thermally induced phase separation and solvent induced phase separation methods. In the present invention, a nematic curvilinear aligned phase (hereinafter referred to as "NCAP") method which allows for the formation of liquid crystal dispersion polymer composite film of PDLC film is employed, although it is not a phase separation method. The NCAP method refers to a method for preparing a PDLC shutter by dispersing and emulsifying a nematic liquid crystal capsule in a water-soluble polymer solution, applying the emulsion onto a conductive transparent plate, drying the emulsion to form a PDLC film, and then laminating another conductive transparent plate. The liquid crystal droplets are neither linked to each other nor dispersed in a regular size and show a very high contrast ratio.
The NCAP method used in the present invention is very simple and economical, and may be employed in many applications. The heart of this method is the technology for controlling the size and stability of liquid crystals when they are dispersed in a water soluble polymer. Therefore, the dispersion technology plays an important role in the properties of the finally completed film. Some researchers have reported employing water soluble copolymers having some hydrophobic group(s) in oil-in-water systems in order to increase the dispersion stability of non-ordered low molecular weight molecules. However, research regarding the emulsification of liquid crystals in water soluble polymer solutions has not yet been conducted on a large scale, and this subject is interesting in view of both theoretical and practical use.
In the related art of preparing PDLC films, the use of dispersing agents is essential in order to improve the dispersibility of liquid crystals in a hydrophilic polymer matrix. Unlike oil-in-water systems, the dispersion of nematic liquid crystals necessitates these dispersing agents to increase the stability of the liquid crystals because the liquid crystals have an ordered structure and a relatively high viscosity.
However, PDLC films prepared by the above methods have the following demerits. Chiefly, the response rate is slow. After removing the electrical field, it takes a long time for liquid crystals to be restored to their original position. In addition, the liquid crystals of these devices amount to about 80 to 85% by weight of the device, making the manufacturing cost too expensive. In addition, the required dispersing agents may decrease the desirable properties of NCAP film as a display.